Ion implantation in Ni-Ti shape memory alloy thin films

Abstract

The present thesis focuses on the effect of 5 MeV Ni ion irradiation on the microstructure and the thermomechanical behavior in prestrained (e∼4%) martensitic Ti-rich NiTi thin films. At this ion energy, damage is limited to a depth of approximately 2 µm (the ion projected range predicted the Transport and Range of Ions in Matter (TRIM) code). When applied to a pre-deformed 6µm thick film, this technique may be used to excite reversible out-of-plane bending which can perform useful work as an actuator in microdevices. Conceptually, the frustration of the martensitic transformation by ion beam damage creates a differential latent strain between the beam-damaged and undamaged layer upon reverse transformation. Furthermore, the beam-damaged layer acts as an intrinsic bias spring that allows a reversible bending motion during thermal cycling. Thus, the actuator and bias spring can be realized in a single thin film element, using a single and simple planar processing. Due to the potential complexity of the thermomechanical response of the bimorph, it is obvious that detailed information about the effects of high-energy heavy-ion irradiation on microstructure and transformation characteristics are needed if the performance of these ion-biased bimorphs is to be optimized. Thus, the present thesis work has been undertaken to develop a better understanding of the influence of ion irradiation parameters on thermally-induced cyclic deflection, with a strong emphasis on the development of the ion damaged microstructure. Cross-sectional transmission electron microscopy (TEM) investigations and X-ray diffraction (XRD) studies of the irradiated microstructure showed that both NiTi and Ti2Ni precipitates were readily amorphized with 5 MeV Ni ions at doses above 5×1013 ions/cm2, especially at lower irradiation temperatures, i.e. 1014 ions/cm2, significant bending movements were achieved. However, a loss in the film's reversible bending motion was observed during initial cycling, especially at high temperatures, that was attributed to the relaxation of the high elastic stresses in beam-damage layer due to the structural relaxations within the amorphous material. For use as actuator material, it is recommended that the NiTi thin films be irradiated above a fluence of 1×1015 ions/cm2 for two reasons, (1) the beam-damaged layer has a more homogeneous microstructure, and thus mechanical properties should be more uniform properties, (2) the bi-morph is more thermally stable and has lower cyclic fatigue.